1. Field of the Invention
The present invention pertains to an air convection pile for refrigerating a medium into which the pile is disposed by removing heat through convection from the medium to the colder ambient air.
2. Description of the Prior Art
Current developments in oil and gas pipelining and offshore drilling in the arctic region have brought to light several problems where refrigeration of permafrost and bodies of water is desirable. Natural convection of the heat to the arctic air is among several effective methods likely to be used to efficiently solve such refrigeration problems.
It has been found that settling problems are extremely severe in ice rich permafrost, even when active heat sources are isolated from the ground surface by construction on pile supports and gravel pads. In such situations, additional permafrost protection is necessary to prevent progressive thawing of the ice rich soils. Passive regrigeration techniques, including air convection, can be used to remove heat from the permafrost to the colder winter air. As long as the permafrost can be kept frozen through the seasonal changes, settling will not occur.
Air convection refrigeration can also be used in the building of ice islands by placing air convection piles in a body of water to freeze the surrounding water. These islands can be used as protective barriers against moving ice sheets or used to ground massive free floating ice formations. It is even possible to use ice islands as direct support for oil and gas drilling operations or to assist in building temporary artificial islands from which to drill wells.
Air convection piles are known to be effective passive refrigeration devices. A concentric tube air convection pile was disclosed in French Pat. 475,226, issued in 1915. The major technical drawbacks of the concentric tube air convection pile reside in its geometry.
Designing an economic and reliable pile hood with an automatic, or manual, close-off system for a concentric tube convection pile is difficult. It is desirable to seal the interior volume of any convection pile from the ambient air when the air is warmer than the refrigerated medium adjacent the convection pile. Without such a mechanism, winds can force a flow of warm air through the pile, warming the refrigerated medium.
Additionally, maintaining an acceptable rate of heat removal for a convection pile requires some minimum rate of air flow through the pile. The rate of air flow through a pile basically depends on two factors. First, the flow rate will increase as the temperature difference between the air and the medium surrounding the pile increases. Second, the flow rate will be decreased by the frictional resistance to air flow posed by a pile's flow channels. A good relative indicator of a pile's air flow frictional resistance is its wetted perimeter. The wetted perimeter of a convection pile is calculated from its cross sectional geometry and is the sum of the perimeters of all walls which the air contacts as it moves through the flow channels. The smaller a convection pile's wetted perimeter is, in relation to the cross-sectional area of its flow channels, the smaller the internal friction forces resisting the air flow therethrough will be. Therefore, everything else being equal, the less resistance a pile poses to the flow of air therethrough, the longer that pile can be and still maintain some minimum rate of air flow to thereby achieve the same acceptable rate of heat removal. The geometry of the concentric tube air convection pile has a very large wetted perimeter in relation to the area of its flow channels.
Another serious drawback of the concentric tube convection pile is its annular flow channel's susceptibility to plugging by ice rings. During the summer months, moisture will condense on the inside surface of the upper end of a pile which is exposed to the ambient air. This condensate runs down the inner wall until it is frozen by the medium surrounding the pile. An ice ring forms at that point. Convective circulation can probably dissipate such an ice ring by sublimation within a relatively short time, possibly just a matter of weeks; but if an ice ring blocks or restricts the annular flow channel, so that no circulation can take place, then sublimation of the ice ring can take possibly months and maybe even seasons or years. There, therefore, exists a need for air convection apparatus to alleviate the foregoing problems.